Minor refactorings and code style changes (#807)

* Made minor changes in some patterns such as removed throws clause where not needed, changed incorrect order of arguments in assertEquals

* Minor refactorings and code style changes. 1) Removed several use of raw types 2) Removed unnecessary throws clauses 3) Used lambda expressions wherever applicable 4) Used apt assertion methods for readability 5) Use of try with resources wherever applicable 6) Corrected incorrect order of assertXXX arguments

* Removed unused import from Promise

* Addressed review comments

* Addressed checkstyle issue

callback/src/main/java/com/iluwatar/callback/App.java

@@ -41,12 +41,7 @@ public class App {
    */
   public static void main(String[] args) {
     Task task = new SimpleTask();
-    Callback callback = new Callback() {
-      @Override
-      public void call() {
-        LOGGER.info("I'm done now.");
-      }
-    };
+    Callback callback = () -> LOGGER.info("I'm done now.");
     task.executeWith(callback);
   }
 }